The present invention relates to protective relays for use in AC electrical power distribution systems and more particularly to distance relays for detecting faults within a protected zone of an AC power transmission line.
Distance relays are well known in the art of protective relaying. For example, reference may be had to "The Art and Science of Protective Relaying", Mason, published by John Wiley & Son, Inc. (1956), particularly to chapter 14. Phase distance relays are utilized in protective systems for AC power transmission lines to detect phase faults or phase to phase to ground faults within a protected zone or portion of the transmission line. Phase distance relays are utilized to detect faults involving more than one electrical phase and if such a fault is detected within the zone or reach of the relay, the phase relay will generate a signal which will cause the operation of a three pole circuit breaker to trip all three phases of the transmission line.
As indicated above, each particular phase relay should only detect faults within its protected zone or reach. The parameters of a phase relay are commonly selected to correspond to the parameters of the transmission line. For example, the parameters are selected to provide a forward reach that may correspond to the forward distance of the zone of the transmission line under the protection of the particular relay. It is desired that the phase relay operate within its selected forward reach and thereby confining the protection of a particular phase relay to a selected zone within the protected system.
The desired operation of a phase relay may be hindered by the inability of the relay to distinguish between internal and external faults. In some cases, this inability is characterized as "overreaching" of the relay. Overreaching is that condition where the relay detects a fault which occurs beyond the end of the protected zone and generates a signal as if that fault had occurred within the protected zone.
In distance relays utilizing energy comparison, a circuit breaker trip signal is generated when the output signal of an operational amplifier, having a feedback circuit comprising a resistor and a capacitor connected in parallel, and an input signal, which is related to the magnitude of an operate signal minus the magnitude of one or more restraint signals, exceeds a predetermined level. Such an energy comparison circuit in a relay is disclosed in my co-pending U.S. patent application Ser. No. 120,078 entitled "Negative and Zero Sequence Directional Overcurrent Unit for AC Power Transmission Line Protection", Ser. No. 120,078, which application is filed concurrently herewith; which is assigned to the same assignee as the present application; and which is incorporated into this detailed description as if fully set forth herein. In this type of protective relay, the operate signal is a full wave rectified AC signal which, along with a full wave rectified AC and other DC restraint signals, are applied to the inputs of a summing amplifier. The restraint signals have a polarity which is opposite to that of the operate signal; therefore, the output of the summing amplifier is a signal which has a magnitude substantially equal to the algebraic sum of the operate and restraint signals.
The rectified sine wave which comprises the operate signal is primarily a DC signal with harmonics of the fundamental frequency of the full wave rectified AC signal, where the second harmonic is much larger than the other harmonics. The harmonics of the fundamental, and principally the second harmonic, make a substantial contribution to the magnitude of the instantaneous level of the operate signal. When the magnitude of the instantaneous level of the operate signal is larger than it should be as a result of the actual fault conditions, the difference between the operate and restraint signals will also be larger thereby resulting in a larger signal applied to the integrator/level detector combination. As a result, an additional contribution to the magnitude of the operate signal due to harmonics of the fundamental frequency can cause the erroneous generation of an output signal from the level detector. This results in reduced security of the system because the relay is generating a trip signal for faults which occur outside of its predetermined reach.
Accordingly, it is an object of the present invention to provide improved discrimination between internal and external faults thereby increasing the reliability of relay operation and the security of the protection system.
Another object of the present invention is to minimize "overreaching" of distance relays in a protective system for AC power transmission lines.
It is still another object of the present invention to minimize erroneous operation of distance relays due to spurious signals which increase the magnitude of the operate signal in the relay.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.